One of methods of precisely measuring the position and shape of a sample (target) uses an electrostatic capacitance sensor (e.g., see Japanese Patent Laid-Open No. 11-230704). According to this method, the magnitude or change amount of an electrostatic capacitance generated between a sensor probe (electrode) and a target is detected to measure the distance between the sensor probe and the target. The electrostatic capacitance is detected as an AC impedance.
FIGS. 22A and 22B show the arrangement of a measurement apparatus according to related art using an electrostatic capacitance sensor. More specifically, the measurement apparatus comprises first and second electrostatic capacitance sensors (sensor probes) 101 and 102, first and second sensor amplifiers 11 and 12 which are electrically connected to the sensors 101 and 102 via connection cables 103, a controller 113 which receives measurement values from the first and second sensor amplifiers 11 and 12, and an oscillator 114 which outputs in-phase drive currents to the first and second sensor amplifiers 11 and 12. Weak AC currents from terminals 11a and 12a of the sensor amplifiers 11 and 12 are supplied from the sensor probes 101 and 102 to a target 104. A voltage drop by the impedance is measured to simultaneously measure distances “gap” between the sensor probes and the target at a plurality of measurement points on the target 104.
Currents flowing from the first and second sensor probes 101 and 102 to the target 104 flow back to terminals 11b and 12b of the sensor amplifiers via conductors which are set to almost the same potential as the housing ground of the apparatus.
In general, an electrostatic capacitance to be measured is a small value in pF order, and is readily influenced by the stray capacitance. The potential is generally so set as to reduce the influence of the stray capacitance on packaging from a sensor amplifier to a sensor probe and packaging from a target to a ground line.
The electrostatic capacitance sensor is ideally used by coupling a target sufficiently low in impedance to ground at low impedance. For this purpose, as shown in FIGS. 22A and 22B, a chuck 105 which chucks the target 104 is generally formed from a conductor such as a metal, and is grounded. In addition, a mount (base) 106 which supports the chuck 105 is insulated.
The sensor probes 101 and 102 are held by a holding member 107 extending from the surface of the chuck 105 such that the sensor probes 101 and 102 face the surface of the target 104.
In the use of a plurality of electrostatic capacitance sensors, an electrostatic field interference must be prevented by, e.g., setting the distance between sensor probes large enough. If the target has a sufficiently low internal impedance and is grounded at sufficiently low impedance, the interference between the sensors can be substantially ignored.
When the measurement target of the electrostatic capacitance sensor is a semiconductor or the like, the target has a high internal impedance. The target may not be able to be grounded at low impedance. In this case, AC currents flow from a plurality of sensor probes into the internal impedance of the target and the ground impedance which are common impedances. Voltage drops at these portions produce errors in sensors (each sensor is comprised of a sensor amplifier and sensor probe).
In the electrostatic capacitance sensor of FIG. 23, Z3 and Z4 function as impedances common to a plurality of sensors. The first and second sensors interfere with each other, and a voltage drop by a sensor current makes, appearing as a measurement error.
A measurement error by the sensor drive phase and ground impedance in the measurement apparatus of related art will be explained with reference to FIGS. 24A to 24F. The drive currents of the first and second sensor probes are in phase, and almost the sum of the two currents flows as a ground current into a common impedance, generating a voltage drop. The voltage drop appears between the terminals (between the terminals 11a and 11b and the terminals between the terminals 12a and 12b) of the sensor amplifiers 11 and 12, resulting in a measurement error in each sensor.